design: add mid-stack inlining design doc

For golang/go#19348.

Change-Id: Ibf3e3817b35226a33d961e76fedb924e15e37069
Reviewed-on: https://go-review.googlesource.com/38090
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

Author: davidlazar